Method and means for paper pulp filtering

ABSTRACT

Process for the filtration of paper pulp with a device that comprises a rotor and a stator; one of the two, filter (1), has perforations of slot or hole types, and the other, the backwashing element, includes means that are intended to generate pressure pulses that consist in combatting the gradual clogging of the perforations of the filter and the gradual flocculation of the pulp during the creep of the pulp between the rotor and the stator by creating deflocculation zones and/or increasing the relative speed between the pulp and the backwashing elements.

This invention relates to the problems of filtration of paper pulp andmore particularly paper pulp that is obtained from the pulping of oldpaper.

In paper pulp technology, it is known to use various filtration deviceseither to eliminate foreign elements or to classify paper pulp fibersaccording to their length or else to increase the paper pulpconcentration by partial elimination of water.

Among the known filtration devices, filters are present that comprise arotor and a stator, one of the two, called a screen below, performingdie function of filtering and comprising perforations such as slots orholes, the other, called backwashing element below, performing thefunction of backwashing and comprising means intended to generatepressure pulses to prevent the clogging of the perforations. The screencan therefore be stationary or rotating, and, conversely, thebackwashing element is rotating with a stationary screen aid stationarywith a rotating sieve. In addition, the liquid can either flow throughthe sieve from the outside to the inside (toward the axis), so-calledcentripetal operation, or from the inside to the outside, so-calledcentrifugal operation.

In this type of device, the pulp reaches one end of the cylindricalscreen, flows into the space between the cylindrical sieve and diebackwashing filter, and comes out at the other end.

As the pulp advances into the filter, it becomes increasingly thick.

This gradual thickening of the pulp produces several effects: a pulpflocculation effect and a sieve clogging effect.

Even if the pulp that is admitted has previously been welldeflocculated, there is the danger of flocks appearing during itsadvance; the greater the flocculation, the more significant theclogging.

If the backwashing filter rotates, it has a tendency to rotate the pulpwith it. If the backwashing element is stationary, the screen has atendency to rotate the pulp. In the two cases, as the backwashing effectby the pressure pulses that are caused by the backwashing element isdirectly linked to the speed deviation between the backwashing filterand the pulp, the more the pulp advances to the end of the screen, theless effective the backwashing is, while at the same time the screen ismore prone to clogging.

In addition, with a fine-slot screen, the long fibers have more troublepassing than the short fibers, and therefore the proportion of longfibers relative to the short fibers increases with the advance of thepulp, which accelerates both the flocculation process and that of theclogging.

This accumulation of phenomena brings about a gradual reduction inproduction per unit of surface area of the screen as the pulp advancesin the filter.

Many patents have had as their object to combat this gradual reductionof the productivity of the filter by intervening, during the travelalong the screen, so as to create strong deflocculation zones and/orzones for greatly slowing the pulp.

In particular, from 1968 in her French Patent 1,539,816, the applicantproposed using obstacles whose purpose is to break the componentparallel to the surface of the sieve from the speed of the liquid.

U.S. Pat. No. 4,383,918 proposed using obstacles that create turbulencebetween the screen and the rotor.

Patent FR 2,613,390, also in the name of the applicant, proposedseparating the screen into several portions and introducing water fordilution between the portions of the sieve.

English Patent GB 2,222,967 proposed installing two sieves that areseparated by stationary flanges placed between the surface of the screenand the rotor.

All of these devices have gradually improved the operation of filtrationdevices that consist of a sieve and a backwashing filter bait in a waythat is still inadequate.

According to this invention:

The zone where the speed deviation increases can be one or more baffles;

The zones for deflocculation and increasing the speed deviation can becombined.

This invention relates to a filtration device or purifier of the typethat comprises a screen of cylindrical shape and a coaxial rotor that isequipped with blades that produce pressure fluctuations to prevent theclogging of said screen; whereby said screen and said rotor comprise atleast two portions of approximately equivalent diameter, whereby theseportions are separated from one another by means whose object is tobreak the component from the speed of the liquid, which is parallel tothe surface of the screen, and to create turbulence characterized by thefact that said means are baffles that consist of, on the one hand,cavities that are placed between the portions of the screen, whereby thebottom of said cavities is optionally farther from the rotor than thesurface of the screen; and, on the other hand, annular deflectors thatare carried by the rotor opposite each cavity; such that it is necessaryfor the majority of the liquid flow to pass through the cavities.

This invention can also comprise all or part of the followingarrangements, taken together or separately:

a The screen consists of a stack of rings,

b The screen consists of a grid that is obtained by juxtaposing barsthat are either parallel to the axis of rotation of the rotor orperpendicular or inclined,

c The screen consists of a juxtaposition of grids as described in thepreceding paragraph, whereby these grids are separated by rings,

d The screen consists of a cylindrical sheet-metal plate that isperforated with holes or slots,

e The screen consists of a stack of cylindrical pieces of sheet metalthat are perforated with holes or slots,

f The cavities are parallelepipedic hollow volumes,

g The cavities are cylindrical hollow volumes,

h Some cavities can be connected at the bottom to a dilution intakewhich can be ensured by water or pulp that is less concentrated than thefiltered pulp.

The purpose of this invention is also the use of this device withregulating means that make it possible:

a) either to adjust the rotor speed based on the pressure differentialof the pulp between the inlet of the device and at least one of theaccepted outlets;

b) or to adjust the rotor speed based on the flow rate or rates in atleast one of the outlets for accepted products;

c) or to adjust the flow rate of the output for rejects from the devicebased on the accepted flow rates and dilution flow rates that aremeasured to regulate the rate of rejects from one or more portions ofthe screen;

d) or by combining two or three of these regulating means.

The purpose of this invention is also the use of means that aredescribed above for the filtration of the paper pulp.

By way of nonlimiting examples and to facilitate the understanding ofthe invention, the accompanying drawings are shown;

FIG. 1: A first embodiment that comprises a slowing zone obtained withbaffles.

FIG. 2: A cutaway view along AA of FIG. 1.

FIG. 3: A variant embodiment of FIG. 1.

FIG. 4: A second variant embodiment of FIG. 1.

FIG. 5: A second embodiment according to the invention that combines adeflocculation zone according to FIG. 1 and a slowing one according toFIG. 1.

FIG. 6. A third embodiment that uses the slowing means of FIG. 1, in acentripetal filter.

FIG. 7: A perspective view that illustrates a screen according to theinvention.

FIG. 8: A perspective view that illustrates the rotor that is intendedto be placed in the interior of the screen of FIG. 7.

FIG. 9: A partial perspective view that shows the interior of the screenof FIG. 7.

FIGS. 10 and 11: Two partial and large-scale views of FIG. 9.

FIG. 12: A partial view of the interior of a variant embodiment of thescreen of FIGS. 7, 9, 10 and 11.

FIG. 13: A partial view of a variant embodiment of a filtering elementaccording to the invention.

FIG. 14: A partial view of another variant embodiment of a filteringelement screen according to the invention.

FIG. 15: A vertical cutaway view of a filtering element according to theinvention.

FIG. 16: An embodiment of a filtration device.

FIG. 17: A embodiment of a filtration device.

In all of FIGS. 1 to 6, the screen consists of, as was described inPatent EP 0 707 109, a stack of circles 1 with a U-shaped section,pressed against one another by means of two end rings 2 and 3 that areassembled together by tie rods 4 that make it possible to ensureprestressing. The bases of the U are equipped with perforations (whetherslots or holes).

The pulp arrives in the filter by its end that is located at the left ofthe figure (on the side of ring 2) and comes out via its opposite end(on the side of ring 3) by circulating in space 6 that is locatedbetween backwashing element 5 and the screen as indicated by arrow f1.

It should be noted, however, that the invention is not limited to thisparticular screen structure but can be applied to any cylindrical screenas will be described below.

According to the invention, a zone for slowing the speed at which thepulp is entrained is created.

For this purpose, as is illustrated in FIGS. 1 and 2, a series ofbaffles that slow down the rotating movement of the pulp are used in thescreen.

By referring to these figures, it is seen that one or more circles 1 arereplaced by a number of partitions 20, parallel to the axis of thefilter that with the flanges of two rings 1 that frame it constitute anumber of parallelepipedic cavities C that are placed over the entirecircumference of the screen.

Opposite these partitions 20 is placed an annular partition 21 that isperpendicular to the axis of the filter and carried by rotor 5.

Thus, as is shown by arrow f2, the pulp flow is deflected by annularpartition 21 and comes up against partitions 20 that are linked to thescreen and passes through cavities C to return into space 6.

This causes a slowing of the rotation speed of the pulp, and, at thesame time, a stirring of the pulp that has a deflocculation effect and afluidization effect.

Fluidization is defined as a stirred state and a state without flock ofthe pulp that promotes the flow into the openings, holes or slots of thesieve.

The result is that the screen is divided into two portions:

a first portion upstream from ring of cavities C and a second portiondownstream, whereby these two portions are separated from one another byring of cavities C.

The result is also that the rotor is divided into two portions bycircular partition 21.

In FIG. 1 placed in the center of the screen; in FIG. 3, it is placedclose to the end, i.e., where the clogging effects are the mostsignificant because of the thickening of the pulp.

FIG. 4 combines the arrangements of FIGS. 1 and 3.

According to the example that is illustrated in FIG. 5, turbulence iscreated in addition by flanges 10 that are fixed to the backwashingelement and flanges 11 that are fixed to the screen.

These flanges 10 and 11 can be radial or inclined and can haveinclinations in the opposite direction.

Preferably, as is shown in FIG. 5 the shapes of flanges 10/11 arc suchthat they overlap one another.

In the zone where said flanges are found, a violent stirring is thusobtained that carries out a good deflocculation of the pulp.

FIG. 6 shows a centripetal filter that uses the means of FIG. 1. Thesame elements bear the same references.

In this figure, screen 1 is found inside of backwashing element 5. Thepulp arrives via the end of the screen and circulates in space 6 that islocated between screen 1 and backwashing element 5.

In a zone of screen 1, the circles with U-shaped section are replaced bypartitions 20, parallel to the axis of the cylinder, and backwashingelement 5 comprises an annular partition 21.

As in the case of FIG. 1, the pulp is deflected by annular partition 21and comes up against partitions 20.

FIGS. 7 to 17 show a variant embodiment of the device that uses theprocess according to this invention.

FIG. 7 shows a cylindrical screen 30 that consists of bars 31 that areplaced vertically beside one another with a slight play (between 0.05 mmand 1 mm) and fixed to horizontal rings 32, 33, 34, 35, 36 and 37.

These rings 32 to 37 are traversed by tie rods 38 and are parallel tothe axis o the cylinder, whereby these tie rods 38 are fixed to two endrings 32 and 37, such that the unit is held in place by the clamping oftie rods 38.

FIG. 8 shows rotor 40 which is placed in the interior of the screen ofFIG. 7. This rotor a cylinder that comprises three annular rings 41, 42and 43. In the zones included between the annular rings are placedblades 45, usually called “foils” in the paper pulp industry.

The function of foils 45 is to create pressure/partial vacuum pulsesthat tend to prevent the clogging of the screen.

Rings 41 to 43 are placed on the rotor to be opposite rings 33, 34 and35.

It will be explained below how rings 33, 34 and 35 of sieve 30 work withrings 41, 42 and 43 of the rotor to form baffles.

As seen in FIG. 8, annular rings 41, 42 and 43 define four zones ofrotor 40.

The shape, the number and the inclination of the blades or foils 45 canvary from one zone to the next.

FIG. 9 shows the screen of FIG. 7, seen from the interior.

It is seen that rings 33, 34 and 35 are hollow so as to define cavitiesC.

In this example, as sieve 30 is made of juxtaposed bars 31, cavities Care not made as in the case of FIGS. 1 to 6 where the screen consistedof U's.

As is seen in FIG. 9, rings 33, 34 and 35 consist of a ring thatcomprises a multitude of parallelepipedic cavities that have virtuallythe same shape as cavities C of FIGS. 1 to 6.

Some cavities C of rings 33 and/or 34 also comprise orifices that areconnected to pipes 50 for dilution intake.

FIGS. 10 and 11 are detail views in larger scale of FIG. 9.

FIG. 12 illustrates a variant according to which, instead of usingparallelepipedic cavities C, cylindrical cavities or recessed holes D,whereby some can be connected to pipes 50, are used.

As is illustrated in FIG. 12 (recessed holes D) and in FIG. 13(parallelepipedic cavities C), the annular ring of rotor 41 (42, 43)that works with ring 33 (34, 35) is placed so as to be virtually at thecenter of cavities C or cylindrical holes D.

Arrow F illustrates how the pulp that comes from the zone that islocated below rings 33, 41 is forced to enter into a cavity C or D,whereby this cavity and ring 41 form a baffle that considerably slowsthe speed of the pulp.

It is seen in these figures that bars 31 are held in place by hoops 31a.

FIG. 14 illustrates another variant embodiment in which the sameelements bear the same references.

In this example, sieve 30 is stationary and placed outside of rotor 40that carries foils 45. Ring 41 of rotor 40 is opposite ring 33 of sieve30 so that cavities C of said ring 33 form a series of baffles withannular ring 41.

In this figure, a pipe 50, which is a water pipe, empties into thebottom of cavity C.

This makes it possible to combine the slowing action that is caused bybaffles C with a dilution action of the pulp, which has a tendency tothicken as it passes through the filtration device.

By referring to FIG. 16, it is seen that the filtration elementaccording to the invention may comprise, from top to bottom:

An upper end ring 32 for screen 30,

An intermediate ring 33 that comprises a number of cavities C, some ofwhich are connected to dilution pipes 50,

A second intermediate ring 34, analogous to ring 33,

A lower end ring 37, connected to ring 32 by tie rods, not shown,

An annular ring 41, carried by the rotor, and located opposite cavitiesC of ring 33 to define with it a number of baffles,

A second annular ring 42, carried by the rotor, located oppositecavities C of ring 34.

The filtration element that consists of screen 30 and rotor 40 is thusdivided into three zones X, Y and Z and starts from the bottom.

The pulp that is to be purified arrives at Q at the base of the device.

Zone X comprises an outlet 60 for so-called “accepted” products, i.e.,that have passed through screen 30; zone Y comprises an outlet 61 foraccepted products, and zone Z comprises an outlet 62 for acceptedproducts.

It is quite obvious that the pulp will have a tendency to thicken and toflocculate as it passes from one tone to the next.

As was said, the thickening causes a reduction in backwashing efficiencythat should be compensated for by, for example, a higher speed andtherefore a larger energy consumption. The introduction of water intopipes 50 that empty into certain cavities C or D of ring 34 and ring 33makes it possible to combat this thickening and thus makes it possibleto reduce the speed of the rotor and therefore to save energy.

In its upper portion, the device comprises a cover 63, an outlet R forwaste.

It is possible to adjust the speed of rotor 40 based on the pressuredifferential between inlet Q in the device and at least one of thepressures that prevail in outlet pipes for accepted products 60, 61, 62.

It is also possible to adjust the speed of rotor 40 based on one or moreof the flow rates in ducts 60, 61 or 62.

In one or the other case, this makes it possible to reduce the speed ofrotation of rotor 40 and therefore to reduce the amount of energyneeded.

The screen can be produced in multiple ways, either by stackingU-section rings of FIGS. 1-6, or by stacking grids that are obtained byjuxtaposing bars of FIGS. 12 and 13, or by stacking cylindrical piecesof sheet metal that are perforated with holes or slots of FIG. 15.

Thus, for example, they can consist of plates 70, in the material fromwhich grooves 71 were cut with circular cutters, whereby the slots arethen machined with circular saws.

It should be noted that by using backwashing blades 45 in an inclinedway, a sort of pumping that accelerates the circulation in the device isobtained.

It should also be noted that whereby rotor 40 is divided into as manyportions as sieve 30 has, it is possible with each portion of the rotorto use blades 45 that are different as to their number, their shape andtheir inclination.

In the same way, the different portions of the sieve can have differentconstitutions.

In all of the examples that are shown, the edges of cavities C or D areexactly at the same level as the one of the inside wall of screen 30.

It is possible to use them so that they project slightly into theinterior of screen 30, provided that this does not prevent theintroduction of rotor 40 into screen 30, i.e., that the annular ringssuch as 41, 42, etc., can pass.

What is claimed is:
 1. Filtration device of the type that comprises acylindrical-shaped screen (30) and a backwashing element (40) that isequipped with blades (45) that produce pressure fluctuations to combatthe clogging of said screen, whereby said screen is composed of at leasttwo portions of approximately equivalent diameter that are separatedfrom one another by means whose purpose is to break the component fromthe speed of the liquid, which is parallel to the surface of the screen,and to create turbulence, characterized by the fact that said means arebaffles that comprise, on the one hand, cavities (C, D) that areprovided in an annular ring (33, 34, 35) of the screen that is placedbetween said portions of the screen, and, on the other hand, an annulardeflector (21, 41, 42, 43) that is carried by the backwashing elementopposite each ring of the screen that carries the cavities, such that itis necessary for the majority of the liquid flow to pass through thecavities.
 2. Filtration device according to claim 1, wherein screen (30)comprises a stack of rings.
 3. Filtration device according to claim 1,wherein screen (30) comprises a stack of grids, whereby each grid isobtained by juxtaposing bars (31) that are either approximately parallelto the axis of rotation of rotor (40) or perpendicular to this axis, orinclined.
 4. Filtration device according to claim 1, wherein screen (30)comprises a stack of cylindrical pieces of sheet metal with holes orslots.
 5. Filtration device according to claim 1, wherein the differentportions of screen (30) are constituted in differing ways.
 6. Filtrationdevice according to claim 1, wherein the cavities are parallelepipedic(C).
 7. Filtration device according to claim 6, wherein some cavitiesare connected to a dilution pipe (50).
 8. Filtration device according toclaim 1, wherein the cavities are cylindrical hollow volumes (D). 9.Filtration device according to claim 1, wherein the number, the type,the thickness and the inclination of blades (45) carried by backwashingelement (40) can vary between annular deflectors (41, 42 . . . ). 10.Filtration device according to claim 1, wherein it also comprises a zonefor violent stirring placed at the inlet of the screen.
 11. Filtrationdevice according to claim 6, wherein the turbulence in the violentstirring zone placed at the inlet of the screen is created by flanges(10) fixed to the backwashing element and flanges (11) fixed to thescreen.
 12. Process for use of a filtration device according to claim 1for the treatment of paper pulp.
 13. Process for use of a filtrationdevice according to claim 12, comprising: providing each portion (X, Y,Z) of the filtration device with an outlet for accepted products (60,61, 62); providing each annular ring (33, 34) of screen (30) with adilution intake, providing the filtration device with an inlet Q, awaste outlet R, and manipulating the adjustments of the outlets foraccepted products, of the outlet for waste and dilution intakes viaducts (50), to reduce the rotation speed of rotor (40) so that it is thelowest possible so as to save energy, while maintaining a low flow rateof the waste.
 14. Process for use of a filtration device according toclaim 13, according to which the speed of backwashing element (40) isadjusted based on the pressure differential between inlet (Q) in thedevice and at least one of the pressures that prevail in outlet pipes(60, 61, 62) for accepted products.
 15. Process for use of a filtrationdevice according to claim 13 according to which the speed of backwashingelement (40) is adjusted based on at least one of the flow rates inoutlet pipes (60, 61, 62) for accepted products.
 16. Process for use ofa filtration device according to claim 13, according to which the rateof rejects of one or more portions of the screen is adjusted byadjusting the outlet flow rate of the rejects of waste outlet R bytaking into account accepted flow rates and dilution flow rates. 17.Process for use of a filtration device according to claim 14.